Semiconductor integrated circuits may be fabricated by etching and vapor depositing certain materials in layers on a silicon wafer to form desired circuit patterns. In an existing fabrication process, the integrated circuits are formed from silicon wafers etched in a vacuum plasma reaction chamber.
A common etching technique employs microwave energy to induce a plasma in a quartz containment tube that is located "upstream" of the reaction chamber. The plasma flows "downstream" through the tube and into the reaction chamber, where the wafer is located and the etching/vapor deposition process ultimately takes place.
More precisely, a microwave generator imparts microwave energy into the "upstream" portion of the containment tube. A combination of process gases are contained in the tube. The quartz tube conducts the activated gases "downstream" to the plasma reaction chamber and the wafer. FIG. 1 illustrates a side view of a typical "upstream" section of a microwave-induced plasma reaction system.
Referring to FIG. 1, an "upstream" section 10 of a microwave-induced plasma reaction system is shown. An etchant chemical in gaseous form (e.g., SF.sub.6 or fluorine gas) flows from an external gas supply (not shown) through input gas coupling 12 to quartz tube 16. The cavity section of microwave generator 18 surrounds a segment of quartz tube 16. Microwave generator 18 imparts microwave energy into quartz tube 16 via the cavity, which activates the etchant gas flowing in the tube. The activated etchant gas forms a plasma that flows through reaction chamber entry seal 17 and into the reaction chamber (not shown), where the wafer to be etched is located. An o-ring 14 seals gas line 12 to the outer surface of quartz tube 16, and another o-ring 19 seals the outer surface of quartz tube 16 to an aperture formed in entry seal 17, which forms a section of a wall of the plasma reaction chamber. The quartz tube and reaction chamber are sealed to maintain a high vacuum (e.g., less than 1-2 Torr).
A significant problem encountered with existing microwave-induced plasma reaction systems such as that illustrated by FIG. 1, is that the very corrosive chemicals activated in quartz tube 16 in the vicinity of the microwave cavity etch the inner walls of the tube. Consequently, quartz particles that are etched from the surface of the tube flow to, and contaminate the surface of, the wafer. Furthermore, the corrosive etchant can eventually eat through the walls of the tube, which can cause the tube to fail and possibly implode. A second, significant problem with the system of FIG. 1 is that the extremely high temperature of the microwave-induced plasma overheats the o-ring seals and eventually causes them to fail. Consequently, the o-ring seals must be replaced often enough to ensure that they do not fail during the plasma reaction process.
One technique that has been considered to solve the above-described quartz etching and contamination problem is to replace the quartz tube with a tube made of a more durable material that does not react readily with fluorine (e.g., a common etchant). For example, materials such as sapphire or certain specialized ceramics do not react readily with fluorine and may be used to form a plasma containment tube. However, the cost to use these materials for a plasma containment tube would be very high, if not prohibitive.
As for the o-ring overheating problem, one potential solution is to encase the regions around the o-rings with water jackets and cool the o-rings with water flowing through the jackets. However, the use of water cooling jackets increases the complexity and cost of the overall process and also creates internal stresses in the cooled areas of the tube near the microwave energy region that increases the chances of tube failure. Moreover, ultraviolet light created by the plasma can impinge on the o-rings, which causes them to deteriorate and eventually fail.
One technique that has been used to minimize the quartz tube etching and contamination problem is to introduce a benign, non-corrosive gas into the quartz tube in the vicinity of the microwave cavity. Consequently, the benign gas is activated to form a plasma by the microwave energy. The etchant or active species of gas is then introduced into the tube "downstream" of the microwave cavity region. Consequently, the "activated" benign gas, in turn, activates the fluorine in the active species of gas.
Specifically, the etchant gas is introduced into the benign gas stream in the quartz tube via a tributary tube disposed perpendicularly to the direction of gas flow in the quartz tube. Consequently, when the etchant gas enters the quartz tube, it flows across the quartz tube and impinges on the opposite wall. This stream of etchant gas rebounds from the opposite wall of the quartz tube, and about half of it flows "upstream" in the tube while the other half flows "downstream". The "upstream" portion of the etchant gas flows into the vicinity of the microwave cavity, where microwave energy is imparted into the gas to activate the etchant chemical. The "downstream" portion of the etchant gas flows to the reaction chamber. Typically, the activated etchant gas in the vicinity of the microwave cavity etches the inner walls of the quartz tube. However, it has been determined experimentally that if the walls of the quartz tube can be cooled just "downstream" of the microwave cavity region, then the quartz etching rate can be significantly reduced. Unfortunately, it is inefficient and thus impractical to place a water jacket around the quartz tube in the vicinity of the microwave cavity, since a significant portion of the microwave energy will heat the water in the jacket instead of activating the etchant gas.